In interleaved analog-to-digital converters (ADCs), an analog input is applied to a common node connected to the inputs of several interleaved channels. The channels operate in an interleaved fashion, e.g., using two or more clocks to control the timing of when each channel samples the input, so that each channel operates in parallel with the other channels, but on a different time sample of the input. The outputs of the channels are then combined to generate a digital output, which is the overall output of the ADC. Mismatches between the channels cause errors in the overall output.
Mismatches can be classified as gain mismatch, timing mismatch, bandwidth mismatch or offset mismatch. The present invention relates to methods and devices for calibrating the first three types of mismatches. Techniques exist for handling offset mismatch, which is not specifically addressed by the present invention. Gain mismatch refers to differences between the gain that the input experiences from the input of a particular channel to the output of that channel. Timing mismatch refers to deviations from the intended sampling instant of a channel. For example, if the input is a sine wave, timing mismatch causes a different part of the sine wave to be seen by the sampling circuit of the channel than the channel is supposed to. This is essentially a phase offset. Therefore, the terms timing mismatch and phase mismatch are used interchangeably in this application.
Bandwidth mismatch has aspects of both gain mismatch and timing mismatch. Bandwidth is a parameter of RC circuits, usually measured at the 3 dB point. Since the sampling circuits in each channel of an interleaved ADC have switching capacitance, external resistance, and sampling capacitance, each channel can be considered an RC circuit (where RC is the resistance times the capacitance of the sampling network), which is affected by bandwidth. The gain portion of the bandwidth mismatch arises from the fact that attenuation is affected by bandwidth (attenuation increases along with the frequency of the input signal). The attenuation directly results in amplitude changes, and therefore gain changes. The timing portion of the bandwidth mismatch arises from the fact that changes in the RC time constant of the channel result in timing errors (e.g., sampling at the wrong instant or outputting at the wrong instant).